In preparation of a silver halide emulsion, a dopant (i.e., substances other than silver and halogen ion) is sometimes introduced into silver halide crystals, which has been well known as a doping technique. The doping technique of transition metals (III to XII groups in the periodic table) has particularly been studied to improve a silver halide emulsion. For example, U.S. Pat. No. 2,448,060 (Trivelli et al) describes that the silver halide emulsion is sensitized with a transition metal compound of R.sub.2 MX.sub.6 (in which R is hydrogen, an alkaline metal or ammonium group; M is trivalent palladium or platinum; and X is halogen). The metal compound may be added to the emulsion at any stages of preparation, namely on or before precipitation of silver halide, on or before the first ripening (physical ripening), or on or before the second ripening (chemical sensitization).
A transition metal compound can be added to the silver halide emulsion at the stage of grain formation. The compound may also be added to the emulsion after precipitation of silver halide grains. However, there is a considerable difference in a photographic effect between the former and latter additions. In the former addition, the transition metal of the compound is introduced into the silver halide crystal as a dopant. Therefore, the transition metal can effectively change the photographic properties of the emulsion, even if a small amount of the compound is used. On the other hand, the transition metal is adsorbed on the surface of the silver halide grains in the latter addition. In this case, a relatively large amount of the transition metal compound is required to change the photographic properties of the emulsion to the same extent as the former addition, since the function of the transition metal to the silver halide grains is inhibited by a protective colloid. Accordingly, it is difficult to obtain a satisfactory photographic effect, if the transition metal is added to the emulsion at the stage of chemical sensitization. Therefore, the transition metal has been usually added as a dopant to the emulsion at the stage of silver halide grain formation. As is described above, metal doping (the former addition) is different from metal sensitization (the latter addition).
Research Disclosure, vol. 176 (December 1978) describes the technical difference between the metal doping and the metal sensitization. In more detail, the chapter IA of the item 17,643 discloses transition metal compounds, which may be added to the emulsion at the stage of precipitation of silver halide grains. On the other hand, the chapter IIIA discloses transition metal compounds, which may be added to the emulsion added during chemical sensitization are described in Chapter IIIA of that publication.
U.S. Pat. No. 3,790,390 discloses use of simple salts of metals or hexa-coordinated complexes thereof having six cyano ligands. The metals are selected from the fourth period in the periodic table (e.g., iron, cobalt, nickel). U.S. Pat. No. 3,790,390 also discloses use of simple salts of ruthenium, rhodium, palladium, osmium or iridium or hexa-coordinated complexes thereof having six halogen ligands. U.S. Pat. No. 3,790,390 further describes silver halide emulsions containing hexa-coordinated complexes of iron(II), iron(III) and cobalt(III).
U.S. Pat. No. 4,126,472 discloses use of iridium as a dopant attached to the surface of silver halide grain or as a surface modifier for silver halide. According to the description of U.S. Pat. No. 4,126,472, silver halide emulsion is sensitized in the presence of a water-soluble iridium salt. The amount of the salt is 10.sup.-6 to 10.sup.-4 mol per 1 mol of silver halide. However, U.S. Pat. No. 4,126,472 is silent with respect to hexa-coordinated cyano complex.
European Patent No. 0,242,190 describes that high intensity reciprocity law failure is reduced in a silver halide emulsion containing silver halide grains, which are formed in the presence of rhodium(III) complex having 3, 4, 5 or 6 cyano ligands.
U.S. Pat. No. 3,690,888 discloses a process for preparing silver halide containing multivalent metal ions. In the process, silver halide is formed in the presence of protective colloid mainly comprising acrylic polymer. U.S. Pat. No. 3,690,888 that the multivalent metal ions include bismuth, iridium, lead and osmium. However, U.S. Pat. No. 3,690,888 is silent with respect to hexa-coordinated cyano complex.
The above-mentioned publications do not disclose that the ligands are introduced into a grain together with the transition metal. Further, they are silent with respect to regulation of the ligand and the effect of the transition metal complex.
European Patents No. 0,242,190 and No. 0,336,426 and Japanese Patent Provisional Publications No. 2(1990)-20853 and No. 2(1990)-20854 describe that silver halide emulsion having excellent characteristics can be obtained by using hexa-coordinated complex of rhenium, ruthenium, osmium or iridium having at least four cyano ligands. In the emulsions described in these publications, low intensity reciprocity law failure is reduced, and the sensitivity and gradation of the emulsion are stable.
European Patent No. 0,336,427 and Japanese Patent Provisional Publication No. 2(1990)-20852 disclose a silver halide emulsion of a controlled sensitivity. In the emulsion, low intensity reciprocity law failure is reduced by using a hexa-coordinated complex of vanadium, chromium, manganese, iron, ruthenium, osmium, rhenium or iridium having nitrosyl or thionitrosyl ligand.
European Patent No. 0,336,689 and Japanese Patent Provisional Publication No. 2(1990)-20855 also disclose a silver halide emulsion of a controlled sensitivity. In the emulsion, low intensity reciprocity law failure is reduced by hexa-coordinated rhenium complex, which has six ligands selected from halogen, nitrosyl, thionitrosyl, cyan, water and thiocyan.
Japanese Patent Provisional Publication No. 3(1991)-118535 discloses a hexa-coordinated transition metal complex having carbonyl group as one of the ligands. Further, Japanese Patent Provisional Publication No. 3(1991)-118536 discloses an emulsion containing a hexa-coordinated transition metal complex in which two of the ligands are oxygens.
In the above-described publications, it has been considered that the hexa-coordinated transition metal complex doped in the silver,halide crystal be sited at the space of seven vacant lattice points corresponding to a silver atom and six halogen atoms adjoining said silver atom. In contrast, it has been generally presumed that a simple ion or atom of the transition metal be introduced into a silver halide grain.
In all the above-described publications, gelatin is used as protective Colloid when the silver halide grains containing a metal Complex are formed. T. H. James, "The Theory of the Photographic Process" (4th ed., Macmillan, Chapter 2, pp. 71-72) describes an interaction between metal and gelatin. According to the description, noble metals such as gold and platinum or heavy metals such as iridium interact with gelatin to form complex salts or to be reduced. Therefore, in the above-mentioned doping techniques of transition metal complex, unexpected effects (for example, oxidation or reduction of the transition metal, exchanging or decomposition of ligands) have been caused by the interaction between the transition metal complex and gelatin. It has been difficult to control the interaction satisfactorily.